1. Field of the Invention
This invention in general relates to photography and in particular to a camera accessory by which a camera of the type having an automatic exposure control system designed for use with a film with a given speed can be adapted for use with a film with a faster speed.
2. Description of the Prior Art
Photographic cameras of the type which include systems by which exposure of a film is automatically controlled in a predetermined manner are well-known in the prior art. In general, such exposure control systems usually consists of an electromechanical shutter arrangement which is controlled by the output of a scene brightness measuring system in such a way that aperture and/or shutter speed are set in accordance with a film's given ASA speed rating and what the scene brightness measuring system judges the "average" scene brightness to be.
Some camera automatic exposure control systems, such as those described, for example, in U.S. Pat. No. 3,446,129 issued to John P. Burgarella on May 27, 1969 and entitled "Exposure Control Apparatus For Photographic Camera"; U.S. Pat. No. 3,848,985 issued to Stewart Bennet on Nov. 19, 1974 and entitled "Automatic Exposure Control System"; and U.S. Pat. No. 4,045,806 issued to Seymour Ellin on Aug. 30, 1977 and entitled "Extended Range Exposure Control System", are structured to provide the camera with the capability of being used with a number of similarly packaged films having different speed ratings. In such "multi-speed" systems, a parameter of the exposure control system is modified either in response to a manual input by the photographer indicating what the speed of the film is or automatically by inserting a specially designed film container which is coded in such a way that the camera control system automatically recognizes what the speed of its film is.
However, other automatic exposure control systems, such as those described, for example, in U.S. Pat. No. 3,641,891 issued to John P. Burgarella on Feb. 15, 1972 and entitled "Exposure Control System"; U.S. Pat. No. 3,744,385 issued to John P. Burgarella et al. on July 10, 1973 and entitled "Control System For Photographic Apparatus"; and U.S. Pat. No. 3,975,744 issued to Bruce K. Johnson et al. on Aug. 17, 1976 and entitled "Automatic Exposure Control System", are designed for use with a single film to given speed and do not have any means by which they can be easily modified to accept a film with a speed rating other than the one for which they are designed except in a limited way through the use of exposure trim arrangements as explained below.
Exposure trim arrangements are well-known in the art and operate to provide a photographer with some discretionary control over the exposure delivered by an automatic exposure control system. Although automatic exposure control systems, whether of the single speed or multi-speed type, have remarkably good exposure performance for the majority of scene lighting conditions normally encountered in photographic work, certain types of scene lighting situations--particularly those termed "high contrast"--do exist which require an exposure which is different from the "average" which would normally be predicted by the automatic exposure control system scene brightness measuring system. Therefore, when an anomalous scene lighting situation, like a strongly back lit scene is encountered, the scene brightness measuring system will predict an "average" exposure which for back-lit conditions will be less than that necessary to capture the more important details of the scene. The effect of such underexposure usually causes the primary subject of the picture to be darker than desired.
Those skilled in the photographic arts have recognized that automatic exposure control systems have this peculiar problem in dealing with "high-contrast" lighting situations and have alleviated the problem by providing the aforementioned exposure "trim" arrangements by which a photographer can manually alter the response of the scene brightness measuring system and thus its output which, in turn, causes the film to be exposed either more or less than it would otherwise be if allowed to operate at its "average" or nominal setting. Thus, with these trim arrangements, such as those described in, for example, U.S. Pat. No. 3,832,721 issued to Irving Erlichman on Aug. 27, 1974 and entitled "Lockout Mechanism For Automatic Trim Assembly Return Mechanism" and U.S. Pat. No. 4,007,468 issued to Igor Blinow et al. on Feb 8, 1977 and entitled "Trim Control Apparatus For Photographic Control System", the photographer is provided with a degree of discretionary control over what the exposure will be and can exercise it according to his personal preference and judgment regarding how much departure from the "average" setting is required for the particular scene being photographed.
While the aforementioned manual "trim" arrangements and other which are functionally similar allow for a degree of personal control over exposure to accommodate the anomalous scene lighting condition, such trim arrangements generally could not be used in the "single speed" type automatic exposure control system to modify the response of its scene brightness measuring system in such a way that the "single-speed" system could be used with another film whose speed is different from that for which the system was designed. This is true for two reasons. First, the range of exposure control for such trim mechanisms is generally on the order of plus or minus one to one and a half stops, and if the film whose speed is different from that for which the single-speed system is designed has a speed which falls outside of this exposure control range of the trim mechanism, the trim mechanism becomes useless for purposes of accommodating the different speed film. And secondly, even if the different speed film does come within the exposure control range of the trim mechanism, to use the trim mechanism would defeat its purpose. In other words, the trim mechanism might be used to accommodate a different speed film in the "single-speed" exposure control system, but if this were done, there would be no means for accommodating proper exposure for the anomalous scene lighting situation.
One known simple solution for adapting a "single-speed" exposure control system for use with a film whose speed is slower than that for which the system is designed is to simply place a neutral density filter over the scene brightness measuring system to cause it to increase exposure for a given brightness. However, no such technique is known by which a faster speed film can be used with the "single-speed" exposure control system, and it is therefore a primary object of this invention to provide a solution to this problem.
It is another object of this invention to provide a camera accessory by which a camera having an automatic exposure control system which is designed for use with a film of given speed can easily be adapted for use with a film of faster speed.
Other objects of the invention will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, the combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure.